Pace = Climbing Mountains and Jumping Off Them
Pace can be a tricky skill to learn and reign in. Every writer I know has struggled with it in one capacity or another. Either your work feels like it's dragging and nothing's happening, or everything's happening too fast and you need to slow it down. Pace isn't something you learn overnight. It's a difficult beast to wrestle, but you will defeat it.
I'm one of those writers who writes so fast I almost always have to slow my writing down in the editing stages. You can't have your reader breathless all the time. It's exhausting. But deciding what to add is sometimes trickier than the first draft! Everything you include in a novel has to mean something. Anything that doesn't add something needs to go. So what do you add? Character building is always a great thing to include. Do your characters know each other as well as they should? A heartfelt conversation can add warmth and also give your reader time to catch their breath after some action.
If you find your writing is slow, you might have to read with a careful eye and cut out some of your darlings. Yes, the conversation is beautiful, but is it slowing things down? Do you need it in this moment? Can it be moved elsewhere? Do you need it at all?
Pay attention to every chapter and the way the tension is rising and falling. If it falls too much in one place, something probably needs cutting. If you're too breathless and running away with yourself, add something to slow it down (unless that was the intention or it's the climax).
Looking at the length of your chapters and sentences and paragraphs can work a treat. If your chapters have been the same length the whole way through the novel, wouldn't throwing in a long or short one ramp up the tension in the reader? The same goes for paragraphs and sentences, but this isn't so subtle. Please don't pepper your manuscripts with too many short sentences and paragraphs. (I'm so guilty of this!) If you have too many short sentences and paragraphs, when you add one for effect, it might get lost.
If you've looked at all the things I've said, and the pace still isn't perfect, maybe you should add some build-up. Build-up is almost as important as the action itself. If the action comes out of nowhere, your reader might not react in the right way. It might be too much of a shock to take in. A great writer can have you leaning closer and closer to the page and gripping the book tighter and tighter as you read. This effect is almost always in the build-up.
So whatever it is you need to do to improve your pace, make sure you listen to what you've written and pay close attention. The hardest thing with pacing is deciding what to change.